Nissan model NS50D2 - two of three cylinders not combusting

[George Edward]

Nissan 50HP
model NS50D2
Mfg date: Dec 2000
Serial #63294

Hi Everyone - this is my first post. Thank you for taking the time to read.

The issue is that the top two cylinders are not combusting on this motor with approximately 50 hrs. on it.

I say "approximately", as the key was left on for a couple weeks, so the hours accumulated quickly while sitting on the trailer. When I bought the boat, this twenty-something year old motor had only 3.5 hours on it!

We fired up the motor and pulled the spark plugs wire off the top two spark plugs, and it made no difference as far as the engine running. When we pulled off the bottom plug wire, the engine would die.

We also sprayed carb cleaner into the intake of the top two cylinders while the engine was running, with no response.

Here's what we have done so far:

• Changed spark plugs.
• Swapped wires and coils to the working third cylinder to test. All three coils and wires worked fine.
• Rebuilt all three carburetors.
• Checked for spark. All plugs are firing.
• Checked compression in all three cylinders. All were in the 120-129 range.

We are stumped. We have spark, fuel, and air, but not getting combustion.

Hoping someone in here has some insight.

Thank you again for your time and thoughts.

George

 

[racerone]

If a 2 stroke have you looked into crankcase compression ?----Fuel pump issues?

 

[pvanv]

Revisit the top 2 carbs.

 

[George Edward]

Revisit the top 2 carbs.

Hi Paul - my friend Brian may be weighing in here soon. He is my personal auto mechanic and has been kind enough to take this motor on as a project with me. He has forgotten more than I will ever know, so I will defer to him regarding the post on Saturday from racerone.

Can you help me understand what you mean by "revisiting the top 2 carbs"?

 

[pvanv]

I usually find on the old 40/50D models that perfection is critical on the carbs.

 

[bmwtechguy]

This is Brian. I worked on George's 3 cyl, 50HP outboard on his sailboat. We saw no trash in any of the carbs, spraying brake cleaner through all ports and passages of every part of those carbs. We did not actually rebuild the carbs, but carefully disassembled, inspected, verified, and reassembled. No adjustments made. Those carbs are definitely getting fuel and the 2 plugs and tops of those pistons get wet when it runs on that one bottom cylinder. That bottom spark plug does not really look wet, not does that piston.

 

[fitz73222]

Leaking upper crankshaft seal, ususlly effects just one cylinder adjacent to the seal, but if the leak is bad enough could effect the second cylinder or there is a loss of crankcase pressure or vacuum as mentioned. Look for oil dripping down the block from the upper crankshaft area.

 

[racerone]

Is this a 2 stroke or 4 stroke model ?----If 2 stroke then perhaps there is a reed valve issue.

 

[bmwtechguy]

It is a 2 stroke model. I assume the reed valve is what allows the fuel oil mixture into the crankcase and that goes into the cyl to be compressed and combusted. I could actually see the reed valve behind where the carbs mounted. There's no doubt that some fuel/oil is getting into the combustion chambers of the two cylinders that are not contributing any power. I'm really not a 2-stroke guy, so I prob need to do some research.